Spontaneous strain due to ferroquadrupolar ordering in UCu2_2Sn

The ternary uranium compound UCu$_2$Sn with a hexagonal ZrPt$_2$Al-type structure shows a phase transition at 16 K. We reported previously that huge lattice-softening is accompanied by the phase transition, which originates from ferroquadrupolar ordering of the ground state non-Kramers doublet $\Gamma_5$. A macroscopic strain, which is expected to emerge spontaneously, was not detected by powder X-ray diffraction in the temperature range between 4.2 and 300 K. To search the spontaneous strain, we have carried out thermal expansion measurements on a single-crystalline sample along the $a$, $b$ and $c$ axes using a capacitance technique with the resolution of $10^{-8}$. In the present experiment, we found the spontaneous $e_{xx} - e_{yy}$ strain which couples to the ground state doublet $\Gamma_5$. The effect of uniaxial pressure along the $a$, $b$ and $c$ axes on the transition temperature is also discussed.Comment: 4 pages, 5 figures, submitted to Phys. Rev.

From Mott insulator to ferromagnetic metal: a pressure study of Ca2_{2}RuO4_{4}

We show that the pressure-temperature phase diagram of the Mott insulator Ca$_{2}$RuO$_{4}$ features a metal-insulator transition at 0.5GPa: at 300K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal; at $T \leq$ 12K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. % We compare the metallic state to that of the structurally related p-wave superconductor Sr$_{2}$RuO$_{4}$, and discuss the importance of structural distortions, which are expected to couple strongly to pressure.Comment: 4 pages, 4figure

Interplay between lattice softening and high-Tc superconductivity in La1.86Sr0.14CuO4

High-resolution ultrasonic measurements have been performed on single-crystalline La1.86Sr0.14 in magnetic fields H up to 14 T. We found that by reducing temperature the transverse elastic constant (C11-C12)/2 starts to soften from about 50 K and continues to soften down to Tc(H), even when Tc(H) is reduced to 14 K by magnetic field. The softening turns to rapid hardening by the emergence of superconductivity. This behavior of (C11 - C12)/2 is a consequence of the structural instability of the orthorhombic phase and disappearance of the instability caused by emergence of the superconducting state

複合極端条件を用いた強相関電子系化合物の物性研究

研究期間:平成10-12年度 ; 研究種目:基盤研究A2 ; 課題番号:10304029原著には既発表論文の別刷を含む

Competing interactions and anisotropic magnetoresistance in layered CeTe2

On a single crystal of CeTe2 with a layered tetragonal structure, we have studied the effect of magnetic field on magnetic susceptibility M/B, specific heat C, and electrical resistivity ρ. It is confirmed that this compound orders antiferromagnetically at TN=4.4 K, while ρ(T) shows no anomaly at TN but a sharp peak at Tρ=6.1 K. Below Tρ, M/B rises suddenly for B∥c, the easy magnetization axis, suggesting the onset of a short-range ferromagnetic order. At 2 K, M(B∥c) shows a metamagnetic transition at a small field of 0.06 T from the antiferromagnetic ground state to a field-induced ferromagnetic state. The peak in C(T) shifts from 4.3 K in zero field to 4.0 K in B∥C=0.1 T, and furthermore a shoulder appears at 4.3 K. With increasing magnetic field, the shoulder changes to a broadened maximum, which shifts towards higher temperatures. These observations indicate that the ferromagnetic interaction competes with the antiferromagnetic one even in zero field. A large negative magnetoresistance, MR=[ρ(B)-ρ(0)]/ρ(0), was observed in the vicinity of Tρ. For I∥c, the MR amounts to -25% at 3 T for B∥c, which is twice that for B⊥c. The large MR for I∥B∥c is a result of the increase of the c-axis conduction in the field-induced ferromagnetic alignment of Ce spins. However, the MR for I⊥c is essentially the same for B∥c and B⊥c, suggesting the confinement of carriers within the Te sheet sandwiched by the ferromagnetically coupled CeTe layers

Role of two-dimensional electronic state in superconductivity in La22xSrxCuO4

We have measured out-of-plane resistivity ρc for La2-xSrxCuO4 under anisotropic pressure, c-axis compression, which decreases ρc, reduces Tc drastically, whereas c-axis extension, which increases ρc, enhances Tc from 38 K at ambient pressure to 51.6 K at 8 GPa. We find that the variation of Tc scales as a function of ρc, and that the c-axis pressure coefficient is much stronger than the ab-axis one. These findings imply that Tc depends primarily on the interlayer, rather than the in-plane, lattice parameter